Starter, levain and the final dough
Your starter is the culture you keep and feed. A levain is a portion of it fed specially for one bake — usually the night before — so it's at its most active when you mix. The final dough is the flour, water and salt you add to that levain on the day.
What “20% levain” means
Like water and salt, the levain is written as a percentage of the flour in the final mix. With 1,000 g of flour, 20% levain is 200 g. Tell the calculator how many loaves and how heavy, and it finds the flour that makes exactly that much dough — then everything else from the percentages.
Recipe hydration and overall hydration
A levain is flour and water too, so the dough is wetter than its water line suggests. Take 1,000 g of flour, 750 g of water and 200 g of levain made with equal flour and water. The recipe says 75% water — but counting the levain's 100 g of flour and 100 g of water, the dough is 850 g of water to 1,100 g of flour: about 77%. The calculator shows both.
Feeding ratios
A feed of 1 : 2 : 2 is one part starter to two parts flour and two parts water, by weight. Give it the levain weight the dough needs and it splits it for you: 200 g of levain at 1 : 2 : 2 is 40 g of starter, 80 g of flour and 80 g of water. A bigger ratio, like 1 : 5 : 5, dilutes the starter more, so the levain generally takes longer to be ready.
Stiff and liquid starters
Not every starter is equal flour and water. A stiff starter has less water than flour; a liquid one has more. Enter your starter's hydration and the calculator works out the levain's own hydration from it and the feed — so the dough's water adds up whatever you keep.